Since its premiere at Sundance 2019, Zac Efron has received much praise for his portrayal of Ted Bundy in Extremely Wicked, Shockingly Evil and Vile, which airs on Friday, May 3, 2019, on Netflix. But, the acclaimed actor says he owes it all to his co-star, Lily Collins, whose performance as Bundy’s ex-girlfriend, Liz Kloepfer made him step up his game.

The Netflix movie is a unique take on the story of the infamous serial killer, who murdered at least 30 women in the 1970s. Based on Kloepfer’s memoir, The Phantom Prince: My Life with Ted Bundy, the film chronicles the life of a serial killer through the eyes of a loved one that struggled to believe his role in such heinous crimes. And, although some critics claim the biopic romanticizes a murderer, Efron and Collins’s performances offer insight into the relationship between a killer and his beloved girlfriend.

While promoting the film at Sundance 2019, Zac Efron praised his co-star, Lily Collins for her portrayal of Liz Kloepefer. “I remember a few days into filming, I just came to work to watch on set because [Lily] was having scenes with Haley Joel Osment, and I was watching her performance and I was blown away,” he told E! News. “She was bringing it, and this was, like, the fourth or fifth day of filming. I was like, I really need to step up my game. Lily is killing it,” he added.

Later, in another interview, Efron reiderated his thoughts on working with Lily to the publication. “She has a very unique way of looking at the work — it’s not work, it’s exploration. She dives really deep into her characters,” he told E! News. “On the first day of set I did my first scene with Lily and I remember the first take the first thing I thought after Joe [Berlinger, the film’s director] said ‘cut’ was like, ‘I need to step up my game. Lily is crushing this.’ And she kept doing that the entire movie — always surprising, always amazing. She’s mesmerizing,” he praised.

It’s not surprising that playing Ted Bundy in Extremely Wicked, Shockingly Evil and Vile was difficult for Zac Efron. In fact, the actor struggled with his own mental health while filming. “You have to be careful for your own emotions, your own safety,” he told Movie Web. “This is a movie that I didn’t go too method, I was able to compartmentalize myself and leave him on set, in the jail cell — most days,” he added.

At the European premiere of the Netflix movie, Metroasked how playing a role like Ted Bundy impacted his own mental health. “That was put to the test for this one,” he remarked. “I’ve never played a role in which I have to separate myself. It was almost impossible,” he added. “I would like to say I did that successfully, but I couldn’t,” he added.

Despite his own struggled with the role, Zac Efron has received much praise from both his co-star, Lily Collins, as well as movie critics. “So how is Zac Efron as Bundy? I think he’s startingly good: controlled, magnetic, audacious, committed, and eerily right,” film reviewer Owen Gleiberman shared in Variety. Even those that didn’t take to the film’s portrayal of Ted Bundy raved about Efron. “Despite Zac Efron’s performance, the film’s humorous take on a serial killer as a rock star doesn’t really work,” wrote a Rotten Tomatoes reviewer.